An antenna for transmitting and receiving an electromagnetic wave using a magnetic material (hereinafter referred to merely as a “magnetic antenna”), in which a magnetic field component coming from the outside is allowed to pass through the core (magnetic material), around which a coil of a conductive wire is wound, to convert the magnetic field component into a voltage (or current) induced by the coil, has been widely used in small sized radios and TVs. Such a magnetic antenna is also used in a non-contact object identification device called an RF tag which has recently widely come into use.
To transmit and receive an electromagnetic wave with a higher frequency, a loop antenna free of a magnetic material and including a loop coil having a coil surface parallel to an object to be identified is used in RF tags. When the frequency is much higher (UHF band or microwave band), an electric field antenna (dipole antenna or dielectric antenna) for detecting an electric field component instead of a magnetic field component is widely used in such RF tags.
However, the loop antenna and electric field antenna have the following problems. That is, when such an antenna comes close to a metallic object, an image (mirror effect) is generated on the metallic object. Since the magnetic field of the image has a phase opposite to that of the antenna, the sensitivity of the antenna tends to be lost.
On the other hand, there is also known a magnetic antenna for transmitting and receiving a magnetic field component which comprises a magnetic material as a central core, an coil-shaped electrode material wound on the core, an insulating layer formed on at least one outside surface of the core on which the coil-shaped electrode material is provided, and a conductive layer formed on an outside surface of at least one of the insulating layers (Patent Document 1). The magnetic antenna described in Patent Document 1 can maintain properties required for antennas even when coming into contact with metal articles.
Also, it is known that a plurality of coils are formed on a core and connected in parallel to each other to obtain an antenna (Patent Document 2).
It is also known that in the case where a coil is lap-wound for reducing a size of the coil shape, when bank winding is used therefor rather than ordinary solenoid coil winding, it is possible to minimize increase in parasitic capacitance of the coil (Patent Documents 3 and 4).